In recent years, next-generation power devices with lower energy loss and miniaturization have grown in demand. For example, multiple layers and higher densities have been demanded of semiconductors for electronics. In response to these demands, the following solutions are considered: semiconductor wafers typified by a Si wafer are greatly reduced in thickness, a machining method causing no dislocations or lattice distortions on a work surface or inside the work surface is developed, and a machining method having a surface roughness (Ra) of sub-nm (sub-nanometers) to nm (nanometers) and a degree of flatness of sub-μm (sub-micrometers) to μm (micrometers) or lower on a work surface is developed.
In automobile industry, IGBTs (Integrated Bipolar Transistors) acting as power devices in automobiles are main systems of inverter systems. In the future, it is expected that higher performance and miniaturization of such inverters will further enhance the salability of hybrid cars. Thus it is necessary to reduce the thickness of a Si wafer making up an IGBT to 50 μm to 150 μm, desirably to about 90 μm to 120 μm to reduce a switching loss, a steady loss, and a heat loss. Further, a perfect surface having no dislocations or lattice distortions is formed on the work surface of a circular Si wafer having a diameter of about 200 mm to 400 mm or in an interior close to the work surface, the surface roughness (Ra) is set at sub-nanometers to nanometers, and the degree of flatness is set at sub-micrometers to micrometers, so that yields in an electrode forming process of semiconductors improve and the number of layers of semiconductors increases.
Generally, the machining process of semiconductors requires a number of steps under present circumstances and so on (for example, patent document 1). The steps include rough grinding with a diamond grinding wheel, lapping, etching, and polishing (Wet-CMP (Chemo Mechanical Polishing) using free abrasive grains). In this conventional machining method, an oxidation layer, dislocations, and lattice distortion occur on a work surface. Thus it is quite difficult to obtain a perfect surface. Moreover, the flatness of a wafer is low and the yields are reduced by a break on a wafer during machining or after an electrode is formed. Additionally, in the conventional machining method, it is difficult to reduce the thickness of a wafer as the diameter of the wafer increases to 200 mm, 300 mm, and 400 mm. Thus under present circumstances, studies have been conducted to reduce the thickness of a 200 mm diameter wafer to 100 μm.
In view of the problems of the conventional art, the present inventors have disclosed an invention relating to a precision surface working machine which can efficiently perform a process ranging from rough machining to ultraprecision surface machining including the final ductile mode machining, only with a precision diamond grinding wheel (patent document 2).
In grinding using such a diamond grinding wheel, three main actions including the rotation of the grinding wheel, the feed of a main spindle for supporting the grinding wheel, and the positioning of a workpiece are important. Precise control on these actions enables precision machining. Particularly, in order to consistently perform a process from rough machining to ultraprecision machining only with a single device, it is necessary to accurately control, of the main actions, the feed of the main spindle over a wide range. In conventional grinding, main spindles are frequently controlled by, for example, methods using servomotors. Such methods cannot sufficiently control areas from a low-pressure area to a high-pressure area with high accuracy, particularly in machining on a low-pressure area where ultraprecision machining is to be performed.
Thus in patent document 2, the present inventors have disclosed a precision machine tool for controlling a pressure with a combination of a servomotor and a super-magnetostrictive actuator. In a pressure range of 10 gf/cm2 or larger, the pressure is controlled by a servomotor and a piezoelectric actuator. In a pressure range of 10 gf/cm2 to 0.01 gf/cm2, the pressure is controlled by a super-magnetostrictive actuator, so that rough machining to ultraprecision machining can be consistently performed by a single device. Further, as a grinding wheel for grinding, a diamond cup grinding wheel having an abrasive grain size smaller than #3000 is used.
Moreover, the present inventors have conducted studies in view of the problems of CMP and found that the problems can be effectively solved by using a synthetic grinding wheel which contains compounds reactive to fine abrasive grains and a workpiece. The compounds are fixed by a specific binder. The inventors have disclosed an invention relating to the synthetic grinding wheel in patent document 3. Grinding using the synthetic grinding wheel is referred to as chemical mechanical grinding (CMG).
Patent Document 1
JP Patent Publication (Kokai) No. 2003-251555
Patent Document 2
JP Patent Publication (Kokai) No. 2000-141207
Patent Document 3
JP Patent Publication (Kokai) No. 2002-355763